Method of improving metal powders



United States Patent METHOD OF IMPROVING METAL POWDERS Nils Herman Brundin and Orjan Wiberg, Hoganas, Swe den, and Ernst Geijer, Riverton, N. 3., assignors to Aktiebolaget Hiiganiismetoder, Hoganas, Sweden, a Swedish company No Drawing. Application December 9, 1955, Serial No. 551,90

3 Claims. (Cl. 75-=-.5)

One process when producing metal powder for powdermetallurgical purposes is as a rule a grinding operation through which the powder is given the desired grain size. This grinding causes a work hardening efiect, which afterwards must be removed by a heat treatment through which the compressibility of the powder will be increased. The heat treatment is usually carried out in reducing atmosphere, as a rule hydrogen gas, whereby the proportion of some impurities, especially oxygen and carbon, is reduced in the powder, which also aids in improving the compressibility. By these methods a certain limit value of the compressibility for each quality of powder is reached. A further increasing of the compressibility would mean increased possibilities to manufacture by pressing and sintering denser details, which by means of the increased density have improved tensile strength or a possibility to obtain hitherto used densities at lower pressure, i. e. at lower cost and with less tool wearing respectively.

This invention relates to a method for such an increasing of the compressibility of metal powders. This new method is mainly characterized in that the metal powder after being ground to the desired grain size but before the following annealing is subjected to a cold working or similar compriming action without grinding, suitably to a rolling operation. The rolling is suitably carried out in such a way that a continuous band is obtained as this causes an exceedingly small deformation of the grains. The band is then again comminuted to powder of the same grain size as that powder from which the band was originally rolled. This operation can be carried out in a disintegrator and afterwards no further grinding is needed. The powder obtained in this way is then annealed in a reducing atmosphere.

Example A quantity of iron powder produced from sponge iron 2,791,498 Patented May 7, 1957 by grinding to a grain size of mesh 0.15 mm.) was divided into two lots, one of them being treated in the hitherto common way by directly annealing the powder during 1 hour at 700 C. in hydrogen atmosphere, whereas the other one was first rolled to a band according to the invention and then pulversized to the same grain size as the original powder, whereafter it was annealed during 1 hour at 700 C. in hydrogen atmosphere. Test pieces were pressed from the two powders at a pressure of 60.000 p. s. i. (corresponding to 4.2 kg./mm. and the density of the test pieces was measured. The following result was obtained:

Test pieces from powder treated in the hitherto common way, 6.27 g./cm.

Test pieces from powder treated according to the invention, 6.55 g./cm.

The coldworking or compriming treatment according to this invention can also be used for quite another purpose in which case the following annealing is not necessary, namely for increasing the volume weight of the powder without substantially altering its grain size. Such powder could be used for 'difierent purposes, the most important of which is the manufacture of coated welding electrodes. Iron powder, as is well known, often is incorporated in the coating of welding electrodes for the purpose of increasing the deposit rate and the yield of metal calculated on the unit length of the electrode. By increasing the volume weight of the iron powder used for this purpose it is possible to increase the iron content of the coating to a certain degree Without necessitating increasing the thickness of the coating and thereby the outer diameter of the welding rod.

We claim:

1. A method of increasing the volume weight of metal powder without substantially increasing its grain size which comprises cold rolling the powder in a layer of sufiicient thickness and under suflicient pressure to form a continuous layer and comminuting said layer to a powder.

2. A method as defined in claim 1 in which the metal powder is sponge iron powder of about 100 mesh grain s1ze.

3. A method as defined in claim 1 in which the layer is comminuted to a powder of the same grain size as the original powder.

References Cited in the file of this patent UNITED STATES PATENTS 2,306,665 Schwarzkoph Dec. 29, 1942 

1. A METHOD OF INCREASING THE VOLUME WEIGHT OF METAL POWDER WITHOUT SUBSTANTIALLY INCREASING ITS GRAIN SIZE WHICH COMPRISES COLD ROLLING THE POWDER IN A LAYER OF SUFFICIENT THICKNESS AND UNDER SUFFICIENT PRESSURE TO FORM A CONTINUOUS LAYER AND COMMINUTING SAID LAYER TO A POWDER. 